Only the rear of the slot along the top of the receiver frame is the sighting key. Ignore the rest of the slot in front. You are not supposed to line the slot up along it's whole length.
Sighting the gun is the same as any other. The front blade rests in the rear notch with the top of the blade lined up with the upper corners of the notch.
The guns in .45Colt should shoot fairly close but hopefully a touch low. Part of the idea is that fixed sights are actually adjustable. It's just that it requires the use of a metal file to adjust them and the adjustments only work in one direction. So you want to be sure of what loads you want to use and the method you'll use for holding the gun while shooting.
The SAA guns seem to beg to be shot one handed in the classic movie cowboy method. But you'll find that if you shoot two handed with a strong support hand grip that the gun shoots to a different POI then it would if you shot one handed or two handed where the second hand is perched there mostly for cocking the hammer.
One other thing to consider is that unless you have very small hands these guns are intended to be held low enough on the grips that the web of your hand is at the bottom of the curve of the neck. Don't be tempted to hold really high or you'll find like a few of us have that the gun want's to shoot strongly to the left. With normal to larger hands the proper hold will see your pinky looking for a place to sit. And the best place in under the butt end of the grips.
As you come to grips with the bullet weight and powder charge you're going to use you can then consider working the front blade shorter with a file until the gun is shooting right on your POA. But sneak up on this over a number of sessions and a few hundred rounds. You CAN go back and start over but this involves a new front sight blade being pressed into the notch. Something best done by a gunsmith. So go slow and make the first front blade your ONLY front blade.
And if you don't reload already the cost of .45Colt ammo will sure encourage you to start....